The present invention relates to optical receivers, particularly to a transimpedance amplifier (TIA) within an optical receiver.
In optical receiver systems, a typical configuration employs a single photodiode to convert light signals into electrical current signals. The need to compensate for the photodiode""s average output current arises to prevent a change in the quiescent bias point of subsequent electrical amplifiers. For example, if the single-ended photodiode feeds a differential amplifier, then the photodiode""s average output current will cause an offset in the amplifier. In turn, this offset affects the switching point of subsequent circuits and could, for example, cause subsequent amplifiers to saturate.
To solve this problem, the average photodiode current must be sensed and then neutralized. A conventional solution involves sensing the offset voltage, due to the average photodiode current, at the output of a TIA. A feedback loop that monitors this voltage provides a low frequency current to the input of the TIA that cancels the photodiode""s low frequency current. The drawback of this approach is that a large capacitor is required at the input of the TIA to suppress noise from the feedback loop. Introducing this large capacitance at the amplifier""s input is problematic because it affects the signal path performance at high frequencies.
U.S. Pat. No. 4,792,997: This patent describes a technique to compensate for the xe2x80x9cdark currentxe2x80x9d from a photodiode, or the current from the photodiode when no illumination is present. This is different than compensating for the DC current present when the photodiode is illuminated by an optical data stream. The technique uses a second photodiode that is not illuminated to accomplish the neutralization.
U.S. Pat. No. 5,295,161: This patent describes the conventional method for compensating the photodiode""s low frequency current. The feedback loop is connected between the output of a differential transimpedance amplifier and one of the differential inputs.
U.S. Pat. No. 6,141,169: This patent describes a compensation loop that is also connected in the typical configuration, where a signal is sensed at the output of the amplifier and the compensating signal is fed in at the input of the amplifier. This particular implementation uses a digital feedback loop involving a comparator, loop filter, and charge pump.
Journal of Solid State Circuits, September 2000, xe2x80x9cHigh-Gain Transimpedance Amplifier in InP-Based HBT Technology for the Receiver in 40 Gb/s Optical Fiber TDM Links,xe2x80x9d pp. 1260-1265: This paper describes a transimpedance amplifier that has a conventional feedback loop to compensate for the offset in the input signal current.
A transimpedance amplifier (TIA) is described. In one embodiment, the TIA includes an input coupled to a photodiode that is used to convert a light signal into an electrical signal to be provided to the TIA. The electrical signal includes a data component and a DC component. The TIA also includes a first differential output and a second differential output coupled to an RC circuit, a negative feedback loop, and an equivalent load RC circuit. The negative feedback loop is used to sense a DC offset voltage at approximately the first and second differential outputs. The feedback loop is used to nullify a sensed DC offset voltage at approximately the differential outputs by passing a current through the RC circuit to generate a correction voltage that subtracts from the DC offset voltage.